If the AdWords account you're signed in to does not have access to DS, you'll see a sign in page. Sign in with a Google account that has been granted access to DS. DS will display the agencies and advertisers that the DS account has permission to access.

If the AdWords account does have access to DS, you can sign in to additional accounts in DS, and then switch accounts.

Switch accounts

In the bar at the top of DS, click your current username.
You’ll see the list of accounts that you’re currently signed in to.

Click an account.
If the account has been granted access to DS, DS displays the agencies and advertisers that the selected account has permission to access.

(If you accidentally click an account that has not been granted access to DS, you’ll see a Google sign-in page. You won’t be able to sign in to DS with this account. Instead, click the Sign in with a different account link that appears on the sign-in page and select an account that has been granted access to DS.)

Sign out

If you sign out of any account while using multiple sign-in, you will be signed out of all your accounts. To resume using multiple sign-in, you will need sign in to one account and then use the Add Account option to sign in to your other account(s). Google won't remember your associated account information, so you'll have to enter your username and password the next time you want to add an account.

To sign out:

In the bar at the top of DS (near the notifications icon), click your current username.